What you can run into with injecting CO² into the inlet is a burping filter.

Since all CO² has to pass through all the filter media.. We have little control over how this actualy works out, it might help disolve it beter, it might slowly accumulate and trap bubbles that eventualy rise and collect at the top in the impeller housing. Than since the pumps are not self priming, the impeller will turn in a gass bubble stop displacing water, loosing pressure and water will syphon back down from the outlet hose back into the pump. And than the impeller gets water again and pushes the buble out with a Burp.
If this is happening you should be causious with it, because it depends on the size of the gas bubble if the pump will be able to push it out again. If it can't the impeller will run dry to long building up heat and extra wear. And it can cost pumps lifespan it needs water for lubrication.. If it happens over night when everybody is snorring you might wake up looking at a burned out pump.
This migh be prevented from happening by looking closly at the pump where its outlet is situated, if its for example in the top centre from the canister. Than make sure the pump is absolute level to prevent large gas bubbles forming at the top.
This is a somewhat exaggerated diagram of a cannister not level. But i guess you get the idea. Gass builds up and than releases bubbles into the impeller housing.
Bubbles make the pump burp, bubbles big enough can make the pump stop displacing water and run dry.
Search for Burping Pumps and you'll find some threads about it, even on non co² tanks pumps can burp from gassious accumulation. Than pumping extra gas in, makes the chances of it happening greater.
Larger bubbles can also be trapped in the media and suddenly release if it gets heavy enough.. Gas bubbles have a tension and need to build up before they release. This you can also see with oxygen bubbles under/on plant leaves. It all accumulates to a certain volume before it pops off ad rises to the surface.